I must agree with John here - I'd keep the Windows SPs up to date and never uninstall them (XP updates need SP1 or SP2 these days anyway) - too many security problems. I've never had problems with the SPs (I didn't even have problems with a non-clean upgrade from Win98 to XP/Pro - I just did the compatibility run first and them went hunting for the new drivers/software versions it told me I needed and then the upgrade went without a hitch).

I did have to sort out a Norton update problem with the 2005 (IIRC) version on a couple of machines (mine included), where the installation seemed to be fine but Live Update then didn't recognise half of the things to update. It wasn't made clear that the old version had to be uninstalled first before installing the upgrade (an unusual thing for an upgrade - perhaps an oversight on Symantec's part). It took a bit of messing about to fix it (though the Symantec KB was helpful enough). Later in the year I saw they'd added big stickers on the outside of the packages saying "You must uninstall the old version before installing this". When I got the 2006 upgrade (online) it installed with no problems.

Mick